This invention relates to the cementing of casing in an oil well, and more particularly to the cleaning of mud filter cake from the walls of the borehole during the cementing process.
An important part of the drilling and completing of an oil well is the process of primary casing cementing. This cementing is done immediately after the casing is placed in the hole. Primary cementing serves to separate and prevent migration of fluid between formations and between a formation and the surface, and to protect the casing pipe itself.
Mechanical aids have been developed to center the pipe in the hole, and to remove mud filter cake formed on the face of the formation. in order to assure a good bond between the casing and the formation. If the filter cake is not properly cleaned, the filter cake may contaminate and weaken the cement, or otherwise prevent a tight seal between the casing and the formation, thereby allowing formation fluid to leak into the annular area between the casing and the walls of the borehole and migrate from one formation to another, or from a formation to the surface.
Common cleaning devices developed to remove this filter cake are wire or cable scratchers which physically contact the wall of the hole and scrape or scratch the filter cake loose. They may be designed to be reciprocated or rotated. Examples of these type cleaners are illustrated in U.S. Pat. No. 3,842,906 to Paramore et al.
It has also long been known that if the cement slurry is kept in turbulent flow while mud is being displaced from the annulus, the turbulence itself will assist in removing filter cake. Thus the practices of high mud displacement velocity and movement of the casing during cementing have been developed to improve primary cementing.
Many devices have been developed to create the desired turbulence, such as those shown in U.S. Pat. Nos. 2,295,803; 3,072,195; and 3,196,952. Another such device is sold by the Weatherford Oil Tool Co., Inc. of 901 Chamber of Commerce Building, Houston, Texas, under the trademark "Hydro-Bonder." All of these devices create turbulence by causing a swirling action of the cement slurry against the wall of the borehole. However, these devices only sustain the turbulence over a limited length of the casing.
In many of these devices, scratchers or skirts extend to the wall of the borehole and are in contact with the wall as the casing is run in the hole. Such a configuration means that the cleaning device is subject to wear and damage as the casing is lowered into the hole, and thus the devices may have lost their effectiveness by the time the casing is completely run into the position where it is to be cemented.
Borehole washing devices have also been developed, such as that shown in U.S. Pat. No. 2,503,719, which are inflatable and force fins and spines into the walls of the borehole to take advantage of both scraping action and turbulence to assist in cleaning. These devices, however, are usually removed before the casing is run, thus allowing the filter cake to reform.
The present invention comprises a plurality of resilient cylinders fastened to a metal strap or band and dimensioned to cause Von Karman vortex trails on their downstream side when mud or cement slurry is moved past them. The cylinders are so spaced that when the strap is helically wound around the exterior of a casing, a random pattern is formed.
The cylinders only extend partially into the annulus region, and are resilient so that if they should make contact with the walls of the borehole they will be flexible enough to minimize damage to the device. Thus, the well bore cleaner will still be intact and effective when the casing is completely run in the hole.
Thus, a well bore cleaning device results which may be used with all sizes of well casings, and which creates or intensifies turbulence in the annular flow while mud is being displaced by a cement slurry in the annulus between the well casing and the walls of the borehole. This turbulence is caused by inducing alternately shedding vortices from each cylinder of a plurality of cylinders distributed in a random pattern over the outer periphery of the casing.
A cleaning device also results which is subject to a minimum of damage as the casing is run into the hole so that the device has not lost its effectiveness when the casing reaches its final location. The device additionally sustains the described turbulence over a large portion of the casing pipe because of the increased area over which the cylinders are distributed.